Allways vertical grain and I usually put it in between the spars right down the middle, however front, back or both will work (I have heard that when a twin spar system fails the top spar breaks upward, but most full sized A/C use it in between). The most important thing to remember is it has to be fully glued and fit snug side to side, and if in between, it has to fit snug top to bottom as well. Anywhere that is does not have a tight fit or is not glued well will create a shear point. It can be a bit tedious fitting them but will add a tremendous amount of strength,and is well worth it.